Self-healing and adaptive materials and systems

ABSTRACT

Solid electrolyte and at least one of piezoelectric and thermoelectric materials are incorporated into material systems to provide them with self-healing and adaptive qualities. The piezoelectric and thermoelectric constituents convert the mechanical and thermal energy, respectively, concentrated in critical areas into electrical energy which, in turn, guides and drives electrolytic transport of mass within solid electrolyte towards and its electrodeposition at critical areas to render self-healing and adaptive effects. Material systems incorporating the solid electrolyte but not the piezoelectric and thermoelectric constituents are also amenable to healing and adaptive effects through external application of electric potential for electrolytic transport of mass towards and its electrodeposition at critical areas.

This invention was made with U.S. government support under ContractW911W6-04-C-0024 by U.S. Army. The U.S. government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is generally related to self-healing and adaptivematerials. Particularly, the invention is directed to materials whichcan alter their internal mass distribution in response to stress andtemperature gradients in order to optimally utilize the availablestructural substance in critical areas subjected to stress andtemperature rise.

2. Description of the Relevant Art

Altering service environments as well as damaging effects change thestress and/or temperature distribution within structures. Biologicalsystems such as bone are capable of adapting to changes in stressdistribution through transport of substance towards and its depositionat highly stressed areas. This adaptive/self-healing capability enablesbiological structural systems make optimal use of available materials asnew circumstances evolve. Various efforts have been made to developsynthetic materials which mimic the self-healing/adaptive qualities ofbiological systems.

U.S. Pat. No. 6,518,330 discloses a self-healing material with thepolymeric healing agent stored in microspheres which are dispersedwithin the material systems. Damage (cracking) of the material wouldcause breakage of the microspheres and release of the healing agent,which fills the crack and rebonds the crack faces. U.S. Pat. No.5,790,304 discloses self-healing coatings incorporating sacrificialconstituents which react with oxygen at defects (e.g., cracks and voids)to produce compounds which condense on such defects and thereby restorethe integrity of coating. U.S. Pat. No. 5,965,266 discloses aself-healing high-temperature materials incorporating constituentscapable of reacting with oxygen to produce compounds to plug cracks andmitigate access of oxygen to the core of the material. U.S. Pat. No.4,599,256 discloses a high-temperature material incorporating multipleconstituents which, when exposed to the elevated service temperature atcracks, react with each other to produce compounds which seal thecracks. U.S. Pat. No. 5,738,664 discloses a material incorporating aviscous flowable constituent which can flow into defects to restore theintegrity of the material.

The above inventions rely on damaging effects (e.g., cracks) to eitherrelease the healing agent or to promote chemical reactions (e.g., uponexposure to oxygen or elevated temperatures) which render self-healingand adaptive effects. Unlike the invention described herein, they do notrely on electrolytic mass transport to strengthen highly stressed areas,and they do not convert the destructive mechanical energy concentratedin critical areas to electrical potential and energy which guide anddrive the self-healing/adaptive effects.

SUMMARY OF THE INVENTION

It is an object of this invention to provide solid material systemswithin which substance can be transported for an optimum massdistribution to be realized.

It is another object of this invention to convert the destructivemechanical and/or thermal energy concentrated within critical areas ofthe material into the electrical energy needed to drive the masstransport phenomenon.

It is another object of this invention to convert the stress and/ortemperature gradients within the material into the electric potentialwhich guides transport of mass towards critical areas.

It is another object of this invention to integrate the energyconversion and mass transport capabilities into a material system whichis inherently capable of transporting substance towards critical areasto render self-healing and adaptive effects.

Applicant has discovered that electrolytic transport andelectrodeposition of mass within solid electrolytes can strengthen anddensify areas within which electrodeposition has taken place. Applicanthas also discovered that the piezoelectric effect can generatesufficient electric potential and energy, by conversion of mechanicalenergy, to drive and guide electrolytic mass transport within solidelectrolyte.

According to the invention, there is provided composite materialsincorporating solid electrolyte and at least one of piezoelectric andthermoelectric constituents, which can strengthen and densify highlystressed areas through electrolytic mass transport andelectrodeposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a fiber reinforced composite under stress, where rupture ofone fiber has caused local stress rise in an adjacent fiber.

FIG. 2 shows a carbon fiber which has received a hybrid coatingcomprising a piezoelectric layer and a solid electrolyte layer withdissolved metal salt.

FIG. 3 shows the cross-section of the carbon fiber which has received ahybrid coating comprising a piezoelectric layer and a solid electrolytelayer (with dissolved metal salt).

FIG. 4 shows a carbon fiber with piezoelectric and solid electrolytecoating layers where local stress rise within fiber has promptedpiezo-induced electric potential difference along the fiber surfacewhich, in turn, drives electrolytic phenomena within the solidelectrolyte layer which transport mass towards and electrodeposit it atthe highly stressed area.

FIG. 5 shows a layered composite incorporating piezoelectric, solidelectrolyte, conductive and structural layers, experiencing a localstress rise under concentrated force, with piezo-driven electrolyticmass transport and deposition strengthening the highly stressed areawhere the concentrated force is applied.

FIG. 6 shows a layered composite incorporating piezoelectric, solidelectrolyte, conductive and structural layers, experiencing a localstress rise due to the presence of a manufacturing defect, withpiezo-driven electrolytic mass transport and deposition strengtheningthe highly stressed area around the manufacturing defect.

FIG. 7 shows a cylindrical structural element, made of a layeredcomposite incorporating piezoelectric, solid electrolyte conductive andstructural layers, subjected to a gradient stress system, withpiezo-driven electrolytic mass transport and deposition strengtheningregions within the structural element which are subjected to higherstress levels.

FIG. 8 shows a layered composite incorporating solid electrolyte layer,conductive and structural layers, subjected to a structural damage,where electrolytic mass transport and deposition via external powersupply is used to strengthen the damaged area.

FIG. 9 shows a thermal protection coating on a substrate, withthermoelectric and solid electrolyte layers introduced as coatingconstituents, where a damage to thermal protection coating causes localtemperature rise, with electrolytic mass transport and deposition drivenby thermoelectric effect bracing the damaged area.

FIG. 10 shows the solid electrolyte specimen sandwiched between twoaluminum electrodes which are connected to a DC power supply.

FIG. 11 shows the cathode electrode where electrodeposition of copperhas taken place for the case with solid electrolyte incorporatingdissolved copper salt but no copper filler.

FIG. 12 shows the cathode electrode where electrodeposition of copperhas taken place for the case with solid electrolyte incorporating bothdissolved copper salt and copper filler.

FIG. 13 shows the electrolysis cell comprising a solid electrolyte sheetsandwiched between two stainless steel electrodes.

FIG. 14 shows a piezo-driven electrolysis test set-up where apiezoelectric sheet is subjected to stress in order to generate theelectric potential and charge needed to drive electrolysis phenomenawithin a solid electrolyte.

FIG. 15 shows: (a) solid electrolyte sheet (with dissolved metal salt)prior to piezo-driven electrolysis; (b) the cathode face of the solidelectrolyte sheet after piezo-driven electrolysis, whereelectrodeposition has taken place; and (c) the anode face of the solidelectrolyte sheet after piezo-driven electrolysis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Damaging effects, changes in service environment and manufacturingdefects modify the stress and/or temperature distributions which developwithin materials, with local stress and/or temperature rise occurring incritical areas which govern eventual failure. Material systems canoptimally utilize their available material resources through partialtransport of these resources towards critical areas which experiencelocal stress and/or temperature rise, where the rise in materialconcentration can render strengthening and densification effects tomitigate the initiation or propagation of damage.

Solid electrolytes are solids which can dissolve metal salts.Electrolytic phenomena can occur within solid electrolytes, and can beused to transport structural substance towards and deposit it atparticular locations in order to strengthen such locations. Thestructural substance is present in solid electrolyte in the form ofdissolved salt; additional structural substance can be introduced in theform of metals which are in contact with the solid electrolyte.

The electrolysis phenomena within solid electrolyte can be guided anddriven by the piezoelectric effect. Piezoelectric and thermoelectricmaterials generate electric potential and charge under stress andtemperature gradient, respectively. If piezoelectric and thermoelectricmaterials are in proper contact with a solid electrolyte, the electricpotential resulting from stress and/or temperature gradients can guideand the corresponding electric charge can drive electrolytic phenomenawithin the solid electrolyte to transport structural substance towardsand deposit it at critical areas experiencing stress and/or temperaturerise. The combination of solid electrolyte with at least one ofpiezoelectric and thermoelectric materials can thus be used to developmaterial systems which can partially adapt their internal massdistribution to internal stress and/or temperature systems which arealtered by at least one of damaging effects, changing serviceenvironments, and manufacturing defects.

Material systems incorporating solid electrolyte, piezoelectric,thermoelectric and other constituents can assume diverse configurations.One configuration introduces the solid electrolyte and at least one ofpiezoelectric and thermoelectric constituents as a multilayer coatingsystem on reinforcing fibers in composites. One other configurationcomprises solid electrolyte matrices reinforced with at least one ofpiezoelectric and thermoelectric fibers; yet another configuration is inthe form of laminated composites comprising solid electrolyte, at leastone of piezoelectric and thermoelectric, and other layers.Alternatively, the piezoelectric and thermoelectric constituents couldbe absent, with external power supply used to adjust distribution ofstructural substance within the solid electrolyte constituent of thematerial systems.

FIGS. 1 through 4 present the example configuration where thepiezoelectric and solid electrolyte constituents are introduced as ahybrid coating on reinforcing fibers in a composite system. FIG. 1 showsa fiber reinforced composite comprising reinforcing fibers and thematrix subjected to external stress; rupture of one fiber is shown tocause local stress rise in an adjacent fiber. FIG. 2 shows a lengthsegment of a carbon fiber that has received a hybrid coating comprisinga piezoelectric layer and a solid electrolyte layer with dissolved metalsalt. FIG. 3 shows the cross section of the carbon fiber which hasreceived the piezoelectric and solid electrolyte coating layers. FIG. 4shows the same fiber as in FIGS. 2 and 3 subjected to local stress risealong its length. The stress gradient in piezoelectric material produceselectric potential on the surface of the piezoelectric layer which is incontact with the solid electrolyte. This electric potential driveselectrolytic transport of metal cations within the solid electrolyte andtheir electrodeposition at the highly stressed location along the fiberlength. This electrodeposition strengthens the fiber at the highlystressed location where fiber rupture could otherwise occur. Thisprocess of mass transport towards and its deposition at the highlystressed location would, in the example of FIG. 1, strengthen thedamaged zone of the composite material where fiber rupture has occurred,and could thus mitigate the propagation of an otherwise catastrophicfailure process.

The examples of FIGS. 1 through 4 introduced the self-healing featuresof the invention. FIG. 5 depicts the adaptive features of the inventionin an example where the piezoelectric and solid electrolyte constituentsare introduced as layers within a laminated structural material.Application of a concentrated force in this example, with the laminatedcomposite placed on a flat surface, causes a local stress rise whichdrives electrolytic mass transport and electrodeposition phenomena tostrengthen the highly stressed region under the concentrated force. FIG.6 presents the laminated composite of FIG. 5 subjected to tensilestress, where a local stress rise is caused by a manufacturing defect,and the electrolytic mass transport and electrodeposition phenomenastrengthen the critical area around the defect. FIG. 7 presents acylindrical element made of a laminated composite similar to thatpresented in FIG. 5, with an eccentric load generating an unsymmetricstress distribution; electrolytic mass transport and electrodepositionphenomena in this case tend to normalize the stress distribution andapproach an optimum use of structural materials. Finally, FIG. 8 showsan application where the piezoelectric constituent is not present, andexternal application of electric potential drives the electrolytic masstransport and electrodeposition phenomena within solid electrolyte tostrengthen a damaged location of the material system.

The key applications of the technology introduced above focus onstructural applications where conversion of mechanical to electricalenergy drives electrolytic phenomena which strengthen highly stressedareas of the structure. Piezoelectricity is the specific effect whichconverts mechanical energy to electrical energy in structuralapplications. Another implementation of the technology is in thermalprotection systems where conversion of thermal energy to electricalenergy, via the thermoelectric effect, drives electrolysis phenomenawithin solid electrolyte to transport substance towards and deposit itat locations experiencing elevated temperature in order to enhance localthermal protection qualities. FIG. 9 shows an application where damageto thermal protection coating causes a local rise in temperature of thesubstrate with thermoelectric and solid electrolyte coating layers. Thethermoelectric effects generates electric potential which guideselectrolytic transport and deposition of mass at damaged area in orderto restore the integrity of the damaged protective coating.

INVENTION AND COMPARISON EXAMPLES Example 1

Solid electrolytes were prepared with dissolved metal salt, without andwith fine copper filler. Electrolysis phenomena occurring in the contextof a solid electrolyte, causing electrodeposition of metal at cathode,were verified experimentally.

Materials

Poly(acrylonitrile) (PAN, M_(w)=86,200), ethylene carbonate (EC, 98%),propylene carbonate (PC, 99%), copper (II) trifluoromethanesulfonate(CuTf, 98%), copper powder (3 micron, dendritic, 99.7%), andacetonitrile (99.93%+, HPLC grade) were purchased from Aldrich, and wereused without any further purification. The use of copper slat in thisinvestigation implies that copper is the metal to be ionicallytransported and electrodeposited to render self-healing effects. Avariety of other metals (nickel, etc.) can replace copper in theprocess.

Preparation of Solid Electrolyte without Copper Filler

PAN (1.06 g or 20 mole %), EC (3.6 g or 41 mole %) and CuTf (1.8 g or 5mole %) were weighed into a ceramic crucible and mixed well beforeadding PC (3.4 g or 34 mole %). PC was then added, and the blend wasstirred until thorough dissolution and a mixture of uniform light bluecolor was obtained. The mixture was then heated to 120° C. andmaintained at this temperature for 45 minutes (using atemperature-programmed oven with heating rate of 20° C./min, and totalheating duration of 51 minutes). The mixture was allowed to cool down toroom temperature, and was then vacuum dried for 24 hours, and furtherdried at 60° C. under vacuum for 2 hours. The end product was lightgreen in color, and it was pressed to yield the test specimen.

Preparation of Solid Electrolyte with Copper Filler

The copper salt dissolved in solid electrolyte can act as the source ofmetallic ion to be transported and deposited for self-healing effects.In addition, one can add copper fillers to raise the quantity of metalavailable to render self-healing effects. In order to prepare thePAN-based solid electrolyte incorporating copper filler, first PAN, ECand CuTf were weighed in a ceramic crucible, and mixed well beforeadding PC. PC was then added, and the mix was magnetically stirred untilthorough dissolution (a uniform mixture) was achieved after about 1hour. Different amounts of copper particles were then added to the mixand magnetically stirred until a mixture with uniform light brown/bluecolor was obtained; the intensity of brown color depended on the dosageof copper filler. The mixture incorporated 1.0 g of water for 10% fillercontent. The remaining steps in synthesis and pressing of solidelectrolyte specimens with copper filler were similar to those taken forthe specimen without filler.

Experimental Procedure

The solid electrolyte was tightly sandwiched between two aluminumelectrodes, as shown in FIG. 10, and a constant voltage was applied fora period of three days. After three days, the aluminum electrodes atanode and cathode were inspected visually.

Test Results and Discussion

Since the solid electrolyte has some copper salt dissolved in it, evenwith no copper filler added to the solid electrolyte, indications ofelectrodeposition of copper was observed to occur on the aluminum sheetat cathode, as shown in FIG. 11, with no such deposition observed atanode.

Copper fillers were added to the PAN-based solid electrolyte tocomplement the dissolved metal salt as the source of metal forelectrolysis processes which render self-healing effects. In the case ofsolid electrolyte with metallic filler, dispersed copper fillers as wellas the dissolve copper salt were the sources of copper for theelectrolysis process. FIG. 12 shows the aluminum sheet surface atcathode after application of constant voltage. Electrodeposition ofcopper on aluminum sheet at cathode is apparent in FIG. 12, with no suchdeposition observed at anode.

Example 2

Materials: The materials used for preparation of PVDF-HFP solidelectrolyte included poly(vinylidine fluoride-co-hexafluropropylene)(PVDF-HFP) (pellets, crystalline copolymer, 15% HFP, averageM_(w)˜400,000), ethylene carbonate (EC, 98%), propylene carbonate (PC,99%), copper (II) trifluoromethanesulfonate (CuTf, 98%), andtetrahydrofuran (THF, 99.9+HPLC grade, inhibitor free). The electrodeswere made of 50 micron thick stainless steel shims. The copper salt wasused in this verification study as an example; other metal salts couldreplace the copper salt to yield self-healing and adaptive effects bydeposition of metals with higher performance-to-weight rations thancopper.

Two different solid electrolytes were prepared by varying theproportions of copper salt, EC and PC while keeping the PVDF-HFPpercentage constant. In order to prepare the solid polymer electrolytewith 3% copper ion concentration, PVDF-HFP was dissolved in THF (30% byweight, 3 g) at 60° C. Subsequently, CuTf (1.8084 g), EC (3.5224 g) andPC (1.7865 g) were added to the mix (70% by weight at CuTf:EC:PC ratiosof 1.0:8.0:3.5), and dissolved until a uniform solution was obtained.The solution was cast on a Petri dish, and left at room temperatureuntil all the THF was evaporated. A free standing polymer sheet ofblue/green color was obtained, which was cut into pieces for use inelectrochemical experiments. Since the most common coordination numberof copper is four, each copper ion will bind with four fluorine atoms(FIG. 12 a). This defines the maximum copper ion-to-polymer molar ratioof 2, which guides our efforts to increase the concentration of copperions in PVDF-HFP.

In order to prepare the solid polymer electrolyte with 6% copper ionconcentration, PVDF-HFP was dissolved in THF (30% by weight, 3 g) at 60°C. Subsequently, CuTf (3.6168 g), EC (1.7612 g) and PC (0.89325 g) wereadded to the mix (70% by weight at CuTf plus EC plus PC), and dissolveduntil a uniform solution was obtained. The solution was cast on a Petridish, and left at room temperature until all the THF was evaporated. Afree standing polymer sheet of blue/green color was obtained, which wascut into pieces for use in electrochemical experiments. Since the mostcommon coordination number of copper is four, each copper ion will bindwith four fluorine atoms. This defines the maximum copper ion-to-polymermolar ratio of 2, which guides efforts to increase the concentration ofcopper ions in PVDF-HFP.

Experimental Procedures

In order to validate piezo-induced electrolysis within solidelectrolyte, PVDF-HFP specimens with dissolve copper salt was sandwichedbetween two stainless steel electrodes, as shown in FIG. 13.Piezoelectric (PZT fiber reinforced composite) sheets were thensubjected to repeated stress application, as shown schematically in FIG.14, and the piezo-induced voltage was applied between the electrodes.Current was measured at pico amp precision (between the piezo-setup andelectrodes). The basic elements of the test set-up are depicted in FIG.13. The current flowing through the solid electrolyte was found to be 20μA; a load frequency of 3 Hz was used in this experiment which lasted 18hours. After this period, the solid electrolyte surfaces at anode andcathode were inspected visually, and were subjected to hardness tests(ASTM D 2240) in order to assess any changes in mechanical attributesassociated with electrolytic mass transport and deposition.

Experimental Results

The experimental results provided clear evidence of metal deposition atcathode interface under piezo-driven electrolysis in solid electrolyte.FIG. 15 a shows the solid electrolyte with dissolved metal salt prior topiezo-driven electrolysis. Observation of the cathode and anodeinterfaces of the solid electrolyte after the test, shown in FIGS. 15 band 15 c, respectively, provided clear evidenced for piezo-drivenelectrolysis at cathode. After piezo-driven electrolysis, the solidelectrolyte adhered to the electrode at cathode. The hardness values atanode and cathode after piezo-driven electrolytic mass transport anddeposition were 33.3 and 48.1 Shore A (ASTM D 2240), respectively,compared with a hardness value of 34.0 Shore A (ASTM D 2240) for thesolid electrolyte prior to piezo-driven electrolysis. The resultsindicate more than 40% gain in hardness (representing mechanicalattributes) at cathode where electrodeposition has taken place,confirming the gain in mechanical properties at cathode associated withpiezo-driven electrolysis within solid electrolyte. On the other hand,anode experiences only about 2% loss of hardness, indicating that thelocal gains in mechanical performance at cathode are achieved throughpiezo-driven electrolysis without any major loss of mechanicalperformance elsewhere.

1. Self-healing and adaptive materials and systems incorporating solidelectrolytes with dissolved salts, and at least one of piezoelectric andthermoelectric materials, wherein gradient stress or temperaturedistributions indicating development of critical areas with elevatedstress or temperature levels induce, via at least one of piezoelectricand thermoelectric effects, gradient electric potentials which transportsubstance towards and deposit it at said critical areas by electrolyticprocesses within solid electrolyte, rendering self-healing and adaptiveeffects.
 2. The self-healing and adaptive materials and systems of claim1, wherein at least one of structural, protective and functionalconstituents are incorporated to meet service requirements.
 3. Theself-healing and adaptive materials and systems of claim 1, wherein thesolid electrolytes are at least one of inorganic, organic and compositeion-conducting materials.
 4. The self-healing and adaptive materials andsystems of claim 1, wherein the salts dissolved in solid electrolytesare metal salts, with self-healing and adaptive effects involvingelectrolytic transport of metal cations and electrodeposition of metalsat critical areas.
 5. The self-healing and adaptive materials andsystems of claim 1, wherein the salts dissolved in solid electrolytesare metal salts, and metal fillers are also incorporated into the solidelectrolyte, with electrostripping of metal fillers providing additionalmetal cations to be transported and electrodeposited to renderself-healing and adaptive effects.
 6. The self-healing and adaptivematerials and systems of claim 1, wherein the piezoelectric constituentsare at least one of inorganic, organic and composite piezoelectricmaterials.
 7. The self-healing and adaptive materials and systems ofclaim 1, wherein the thermoelectric constituents are at least one ofmetallic, inorganic, organic and composite thermoelectric materials. 8.Materials and systems that are amenable to externally stimulated healingand adaptive effects, incorporating solid electrolytes with dissolvedsalts and optionally at least one of structural, protective andfunctional constituents, wherein external application of electricpotential to the system transports substance towards and deposits it atsaid critical areas by electrolytic processes within solid electrolyte,to heal damaged or defective areas, or to adapt the material to newservice requirements.
 9. The materials and systems of claim 8, whereinat least one of structural, protective and functional constituents areincorporated to meet service requirements.
 10. The materials and systemsof claim 8, wherein the solid electrolytes are at least one ofinorganic, organic and composite ion-conducting materials.
 11. Thematerials and systems of claim 8, wherein the salt dissolved in solidelectrolyte is a metal salt, with the healing and adaptive effectsinvolving electrolytic transport of metal cation and electrodepositionof metal at critical areas.
 12. The materials and systems of claim 8,wherein the salt dissolved in solid electrolyte is a metal salt, andmetal fillers are also incorporated into the solid electrolyte, withelectrostripping of metal fillers providing additional metal cations tobe transported and electrodeposited to render self-healing and adaptiveeffects.